Results for high heat-flux flow realizations in innovative operations of milli-meter scale condensers and boilers

Innovative realizations of shear-driven boiling and condensing flows have been investigated within horizontal channels (with condensation or boiling on the bottom, horizontal surface). Results from boiling and condensing flows of FC-72 fluid in horizontal rectangular cross-section (2 or 6 mm gap height and 15 or 24 mm width, respectively) ducts of 1 m length are presented. Utilizing a controlled presence of pulsatile mass flow rates, significant enhancements in heat-transfer rates are obtained for these innovative devices at a location within the device length - these enhancements (relative to non-pulsatile conditions) are sometimes >860% for condensing flows and >190% for boiling flows. The paper reports representative time-varying heat-flux values at this location in support of the understanding that this phenomena can potentially be used (in future experiments) to significantly enhance average heat-flux value over the entire length of an innovative boiler/condenser. The reported phenomena arises from superposing relatively fast time-scale (2.5-30 Hz) flow rate pulsations on otherwise steady-in-the-mean flows to beneficially change certain flow variable averages - over the longer time-scale (<0.01 Hz) of practical interest - relative to their values obtained in the absence of externally imposed pulsations. Such externally imposed pulsations may cause standing waves to form on the thin film interface. The observed asymmetric reduction in the mean film thickness (associated with high heat-flux values) is likely due to “stickiness” of wave-troughs resulting from interaction of phenomena associated with several length scales (namely nano-scale, micro-scale, and continuum length scales) and two different time scales - short (less than 0.5 s) and long (greater than 1 min). That is, associated micro-scale flows at wave-troughs interact with and, possibly, de-stabilize the local adsorbed layer (whose thickness may be <200 nm and is exposed to phenomena such as disjoining pressures) on the wetting heat-exchange surface.